The present invention relates in general to pipe bending and, in particular, to a new and useful rotary draw bending apparatus and method which avoids the formation of indentations or humps in the pipe at the end of the bend and resists the bowing of the pipe in back of the bend die, resulting from the application of excessive force on the pressure die.
The technique of rotary draw bending derives its name from the concept of a pipe or tube being clamped, held and rotated around a bend die. During the process, the pipe is pulled or drawn forward. This is different from the so-called compression bending where the pipe is stationary and is wrapped around a form. The minimum tools or dies required for rotary draw bending are a clamp die, a bend die, and a pressure die. The clamp die holds the pipe or tube against the bend die and rotates with the bend die. The pressure die holds back reaction forces to create the bend. All bending occurs within a narrow zone just prior to and slightly following the point where the pressure die is tangent to the bend die. It is within this range that the yield point of the pipe or tube material is exceeded thereby forming indentations or humps.
The machine operators are known to apply excessive force on the pressure die in an attempt to improve the ovality of the tube or pipe within the bend. The application of excessive force may cause the formation of indentations or humps in the pipe at the end of the bend and may cause the pressure die to over travel and actually bow the pipe in back of the bend die. The present invention avoids the formation of indentations or humps in the pipe at the end of the bend and resists the bowing of the pipe in back of the bend die, resulting from the application of excessive force on the pressure die.